The present invention relates to a novel, inventive process for the stereoselective preparation of 2-hydroxybutyrates, in particular of 2-hydroxy-4-phenylbutyrates (HPB ester), and of their precursors.
HPB esters of formula 
wherein A is substituents, m is an integer from 0 to 5 and R is an ester group and wherein the symbol * indicates a chiral centre, are important intermediates for the preparation of pharmacologically effective ACE inhibitors (ACE: angiotensin converting enzyme) which have the following shared structural feature: 
ACE inhibitors belong to the active ingredient group of the antihypertensives and effect after oral administration a competitive inhibition of the so-called angiotensin converting enzyme and thus a lowering of the blood pressure. A particularly preferred HPB ester has the R-configuration.
An important active ingredient is 3-[(1-(ethoxycarbonyl)-3-phenyl-(1 S)-propyl)amino]-2,3,4,5-tetrahydro2-oxo-1H-1-(3S)-benzazepine-1-acetic acid hydrochloride which is known under the INN name benazepril HCl and which is commercially available in diverse oral forms of presentation, e.g. tablets, under the registered trademark Cibacen(copyright) (trademark of Novartis AG, Basel Switzerland). HPB esters can furthermore be used as intermediates for the preparation of other known ACE inhibitors, for example enalapril, cilazapril, spirapril, quinapril, ramipril or lisinopril (INN names). HPB esters can also be used for the synthesis of different types of insecticides.
Many different methods are known for preparing R-configurated HPB esters, for example reductions with micro-organisms or enzymes, enantioselective hydrogenations with homogeneous or heterogeneous catalysts, diastereoselective hydrogenations, reduction with hydrides, reactions with so-called chiral building blocks, enzymatic racemate resolution or racemate resolutions on chiral substrates, or inversion of the S-HPB ester.
These methods are disadvantageous for various reasons, such as
use of costly educts, e.g. 2-oxo-4-phenylbutyric acid
reaction at low concentrations (general problem in the case of biological or enzymatic processes)
high process costs in the case of homogeneous catalysis
maximum yield of only 50% in the case of racemate resolutions.
This invention has for its object to enantioselectively synthesise 2-hydroxybutyrates of the desired configuration, in particular HPB esters, starting from starting materials which are obtainable by simple synthesis. In the narrower meaning, this invention has for its object to enantioselectively synthesise HPB esters having the desired R-configuration without the losses necessitated by racemate resolution.
European patent application No. 206 993 describes the preparation of HPB esters through heterogeneous catalytic reduction with platinum catalysts of xcex1-keto esters of formula 
The hydrogenation can be carried out enantioselectively in the presence of a chiral modifier, e.g. cinchonidine, so that the desired R-form is predominantly obtained. In spite of this possibility, this process is disadvantageous because the xcex1-keto ester must be prepared before-hand, the synthesis of which over several process steps is complicated.
Surprisingly, it has been found that starting from the xcex1,xcex3-diketo esters of formula 
(R1 e.g. phenyl) which are accessible by simple Claisen condensation, or from their tautomeric xcex1-unsaturated xcex1-hydroxy-xcex3-keto esters; 
an xcex1-hydroxy-xcex3-keto ester is obtained by enantioselective hydrogenation in the presence of a suitable chiral modifier, which xcex1-hydroxy-xcex3-keto ester can be crystallised in enantiomerically pure form, for example in the desired R-configuration, in high optical yield and which can be converted to 2-hydroxybutyrate in a subsequent catalytic hydrogenation.
This invention relates to a process for the preparation of compounds of formulae 
wherein
R is hydrogen or an ester group,
R1 is hydroxy, etherified hydroxy, C1-C8alkyl, C3-C8cycloalkyl, phenyl or phenyl which is substituted by 1 to 5 substituents, R2 is hydrogen or C1-C4alkyl and n is 0 or 1, which process comprises enantioselectively hydrogenating an xcex1,xcex3-diketo ester of formula 
wherein R, R1 and R2 have the cited meanings, or the tautomer thereof, with platinum as catalyst in the presence of a cinchona alkaloid as chiral modifier and, if desired, hydrogenating for the preparation of a compound (IA) or (IB), wherein n is 0, one of the obtainable compounds of formulae 
xe2x80x83having the desired configuration with palladium as catalyst.
A preferred embodiment of this invention relates to a process for the preparation of a compound (IA) or (IB), wherein R is e.g. hydrogen, C1-C4alkyl, preferably methyl or ethyl. R1 is hydroxy, etherified hydroxy, for example C1-C4alkoxy, such as methoxy or ethoxy, C1-C4alkyl, for example methyl, ethyl, n-propyl, isopropyl or n-, iso- or tert-butyl, phenyl or phenyl substituted by 1-5 substituents A. R2 is hydrogen or C1-C4alkyl, preferably methyl and n 0 or 1.
A particularly preferred embodiment of this invention relates to a process for the preparation of compounds of formulae 
A is substituents,
m is an integer from 0 to 5 and
R is an ester group, which process comprises enantioselectively hydrogenating an xcex1,xcex3-diketo ester of formula 
wherein A, m and R have the cited meanings, or the tautomer thereof, with platinum as catalyst in the presence of a cinchona alkaloid as chiral modifier and hydrogenating one of the obtainable compounds of formulae 
xe2x80x83having the desired configuration with palladium as catalyst.
The symbols, terms and denotations used in the description of this invention are preferably defined as follows: 
the symbols
and 
in the structural formulae mean that a predominant number of the molecules has the indicated stereochemical configuration at the chiral centre which, according to the nomenclature rules (R,S-nomenclature) of Cahn, Ingold and Prelog, has the denotation R or S.
In compounds (IA) and (IB) the ester group R is preferably a saturated hydrocarbon radical, in particular C1-C20alkyl, C3-C12cycloalkyl, C2-C11heterocycloalkyl, C6-C16aryl, C1-C1-C15heteroaryl or C7-C16aralkyl, which can be substituted by one or more than one substituent selected from the group consisting of C1-C6alkyl, C1-C6alkoxy, C1-C6haloalkyl, C6-C16aryl, carboxy, C1-C4alkoxycarbonyl, C1-C4alkanoyl, xe2x80x94SO3xe2x88x92, ammonium and halogen.
Examples of C1-C20alkyl are methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl and the isomers of pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl and dodecyl. An example of aryl-substituted alkyl is benzyl.
Some examples of C3-C12cycloalkyl are cyclopropyl, cyclopentyl and cyclohexyl. Examples of substituted cycloalkyl are cyclopentyl and cyclohexyl which are substituted by methyl, dimethyl, trimethyl, methoxy, dimethoxy, trimethoxy, trifluoromethyl, bis-trifluoromethyl and tris-trifluoromethyl.
C2-C11heterocycloalkyl preferably contains one or two, and C1-C15heteroaryl one to four, heteroatoms, which are selected from the group consisting of oxygen, sulfur and nitrogen. Some examples of heierocycloalkyl are tetrahydrofuryl, pyrrolidinyl, piperazinyl and tetrahydrothienyl. Some examples of heteroaryl are furyl, thienyl, pyrrolyl, pyridyl and pyrimidinyl.
Examples of C6-C16aryl are phenyl and naphthyl. Examples of substituted aryl are phenyl substituted by methyl, dimethyl, trimethyl, methoxy, dimethoxy, trimethoxy, trifluoromethyl, bis-trifluoromethyl or tris-trifluoromethyl. One example of C7-C16aralkyl is benzyl. Examples of substituted aralkyl are benzyl substituted by methyl, dimethyl, trimethyl, methoxy, dimethoxy, trimethoxy, trifluoromethyl, bis-trifluoromethyl or tris-trifluoromethyl.
Heterocycloalkyl preferably contains one or two, and heteroaryl one to four, heteroatoms, which are selected from the group consisting of oxygen, sulfur and nitrogen. Some examples of heterocycloalkyl are tetrahydrofuryl, pyrrolidinyl, piperazinyl and tetrahydrothienyl. Some examples of heteroaryl are furyl, thienyl, pyrrolyl, pyridyl and pyrimidinyl.
A saturated hydrocarbon radical R is preferably C1-C7alkyl, for example methyl, ethyl, n-propyl, n-butyl, isobutyl or tert-butyl.
Suitable substituents A of phenyl are typically C1-C7alkyl, for example methyl, ethyl, n-propyl, n-butyl, isobutyl, tert-butyl or neopentyl, C1-C7alkoxy, for example methoxy, ethoxy or tert-butoxy, C1-C4alkanoyl, for example acetyl or propionyl, C1-C4alkanoyloxy, for example acetoxy, cyanogen, halogen, for example fluoro, chloro, bromo or iodo, hydroxy, carboxy, C1-C4alkoxycarbonyl, for example methoxy- or ethoxycarbonyl, C1-C7alkylenedioxy, for example ethylenedioxy, amino, C1-C4alkylamino, for example methyl- or ethylamino, di(C1-C7alkyl)amino, for example dimethyl- or diethylamino, C1-C4alkanoylamino, for example acetylamino, carbamoyl, C1-C4alkylcarbamoyl, for example methylcarbamoyl, di(C1-C7alkyl)carbamoyl, for example dimethylcarbamoyl, C1-C4alkanesulfonylamino, for example mesylamino or trifluoromethanesulfonylamino, arenesulfonylamino, for example benzenesulfonylamino or p-toluenesulfonylamino, sulfo, sulfamoyl, C1-C4alkylsulfamoyl, for example methylsulfamoyl, di(C1-C7alkyl)sulfamoyl, for example dimethylsulfamoyl, halogen-C1-C7alkyl, for example trifluoromethyl, hydroxy-C1-C7alkyl, for example hydroxymethyl or 1- or 2-hydroxethyl, or amino-C1-C7alkyl, for example aminomethyl or 1- or 2-aminoethyl.
Two substituents A can form bivalent, bridge-like C2-C6alkylene, C4-C8alkyldiylidene or C4-C8alkenyldiylidene groups, preferably butanediylidene, in particular 2-butendiylidene, which are bound with the phenyl ring to two adjacent carbon atoms and which form with these carbon atoms a bicycle, preferably a condensed biphenyl ring, for example the naphthyl group, which bicycle can be substituted by the cited functional groups or substituents.
Functional groups, for example amino, hydroxy, carboxy or sulfo, can be protected by suitable protective groups, for example trimethylsilyl, tert-butyl, p-nitrobenzyl, phthaloyl etc.
Salt-forming groups, for example carboxy or sulfo, can be present in free form or in salt form, for example in the form of sodium salt. Amino groups and substituted amino groups can be present in free form or in the form of acid addition salts, such as hydrochloride.
The index m is preferably 0, 1, 2 or 3.
In a preferred embodiment of the process, a compound (I Axe2x80x2) or (I Bxe2x80x2) is prepared, wherein m is 0 and R is C1-C4alkyl.
In a particularly preferred embodiment of the process, a compound (I Axe2x80x2) is prepared, wherein m is 0 and R is ethyl.
A chiral modifier contains a basic nitrogen atom which is close to one or several chiral centres, which in turn are bound to a bicyclic aromatic compound. Suitable chiral modifiers are described by A. Pfaltz and T. Heinz in Topics in Catalysis 4(1997) 229-239. Preferred modifiers are cinchona alkaloids which are known under this name and which belong to the group of quinoline vegetable bases which can be isolated mainly from the bark of trees of the cinchona and remijia family. This definition embraces in particular the alkaloids (xe2x88x92)-quinine, (+)-quinidine, (+)-cinchonine and (xe2x88x92)-cinchonidine. The use of (xe2x88x92)-quinine and (xe2x88x92)-cinchonidine results in compounds (III) in the R-form (III A), and the use of (+)-quinidine and (+)-cinchonine results in compounds (III) in the S-form (III B). It is preferred to use (xe2x88x92)-cinchonidine and derivatives thereof.
In a particularly preferred embodiment of this invention, the chiral modifiers used for the preparation of the R-form (III A) are derivatives of the (xe2x88x92)-cinchonidine of formula 
wherein
R is hydrogen, methyl, acetyl, lactoyl or lactoyl etherified by benzyl, and
Rxe2x80x2 is ethyl or hydroxymethyl.
Compound (IV) wherein R is hydrogen and Rxe2x80x2 is ethyl is known under the name 10,11-dihydrocinchonidine (HCd), and compound (IV) wherein R is methyl and Rxe2x80x2 is ethyl is known under the name O-methoxy-10,11-dihydrocinchonidine (MeOHCd), and compound (IV) wherein R is hydrogen and Rxe2x80x2 is hydroxymethyl is known under the name norcinchol.
In a preferred embodiment of the process, the chiral modifier (IV) is 10,11-dihydrocinchonidine (HCd).
The enantioselective reduction is carried out in a manner known per se. The platinum catalysts used can be in the form of so-called polymer-stabilised colloidal metal clusters, such as those described by X. Zuo et al. in Tetrahedron Letter 39(1998) 1941-1944, or are preferably applied to suitable substrates. Examples of suitable substrates are carbon, aluminium oxide, silicium dioxide, Cr2O3, zirconium dioxide, zinc oxide, calcium oxide, magnesium oxide, barium sulfate, calcium carbonate or aluminium phosphate. Aluminium oxide is preferred. The catalysts are activated in a manner known per se with hydrogen at about 200 to 400xc2x0 C. and are then modified with the solution of cinchona alkaloid and impregnated, and/or the cinchona alkaloid is added direct during reduction.
Hydrogenation is carried out in the presence of water or of an organic solvent. It is preferred to use polar and non-polar aprotic or polar protic solvents or mixtures thereof.
Examples of suitable non-polar aprotic solvents are hydrocarbons, for example aliphatic hydrocarbons, e.g. hexane, heptane or petroleum ether, cycloaliphatic hydrocarbons, for example cyclohexane or methylcyclohexane, aromatic hydrocarbons, for example benzene, toluene or xylene.
Examples of suitable polar aprotic solvents are ethers, for example aliphatic ethers, e.g. diisopropyl ether, 1,2-diethoxyethane or tert-butylmethyl ether, cyclic ethers, for example tetrahydrofuran or dioxane, amides, for example dimethylformamide or N-methylpyrrolidone. Particularly suitable are ethers, in particular tetrahydrofuran.
Examples of suitable polar protic solvents are alcohols, for example ethanol or n-butanol.
The process may preferably be carried out in the liquid phase batchwise or continuously, preferably with a catalyst suspension as liquid-phase hydrogenation or in a bubble column or with a formated catalyst in a trickle bed. The reaction can also be carried out in the gas phase with a powdered catalyst in a fluidised bed or with a formulated catalyst in a fixed bed.
The hydrogenation can be carried out in a wide range of temperatures. Advantageous temperatures have been found to be those in the range from room temperature to about 100xc2x0 C., preferably from 20xc2x0 to about 50xc2x0 C.
The hydrogen pressure can vary within a wide range during hydrogenation, for example from 1-200, preferably from 5-100, more preferably from 10-60 bar. Which hydrogen pressure is used depends essentially on the hydrogenation plant available.
The reaction time can vary within wide limits and depends on the catalyst used, on the hydrogen pressure, on the reaction temperature and on the plant used. It can be, for example, in the range from half an hour to 24 hours. Advantageous reaction times are those from about half an hour to two hours.
The isolation of the reaction products is carried out by known methods and is illustrated in the Examples. After separating the catalyst and removing the solvent, the customary separation processes may follow, for example preparative thin-layer chromatography, preparative HPLC, preparative gas chromatography and the like. In a particularly preferred embodiment of this invention, the R-compound (III A) is crystallised from a suitable solvent. Diisopropyl ether has been found to be a particularly advantageous solvent. The R-compound (III A) is obtained in a special optical purity ee of up to 99% by crystallisation from this solvent.
The subsequent hydrogenation of the compound (III A) or (III B) with palladium as catalyst, e.g. palladium (black) or palladium chloride, is carried out in a manner known per se. The palladium catalysts used are applied to substrates. In a preferred embodiment of this invention palladium is applied to carbon.
The solvents used are polar protic solvents, for example ethanol. If required, acid assistants are added, for example organic mono- or polyvalent acids containing more than two carbon atoms, for example acetic acid, propionic acid or malonic acid, mineral acids, for example hydrogen chloride or sulfuric acid, so-called Lewis acids, for example boron trifluoride, or so-called solid acids, such as zeolites or Nafion(copyright) and/or dehydrating agents, for example sodiun sulfate.
The isolation of the reaction products is carried out by known methods and is described in the Examples. After separating the catalyst and removing the solvent, the customary separation processes may follow, for example preparative thin-layer chromatography, preparative HPLC, preparative gas chromatography and the like.
Compounds of formulae 
wherein A is substituents, m is an integer from 0 to 5 and Et is ethyl, are novel and are also a subject matter of this invention.
A particularly preferred embodiment of this invention is the compound (R)-2-hydroxy-4-oxo-4-phenylbutyric acid ethyl ester.
Enantiomerically pure compounds of formula 
wherein R is an ester group with the exception of methyl, are novel and are also a subject matter of this invention.
Enantiomerically pure compounds of formula (III Axe2x80x3) or (III Bxe2x80x3), wherein R is hydrogen or methyl, are known (CAS 61689-31-4 and 61689-32-5).
A particularly preferred subject matter of this invention is the compound 5,5-dimethyl-(R)-2-hydroxy-4-oxohexanoic acid ethyl ester.
This invention also relates to the use of compound I Axe2x80x2 having an R-configuration for the preparation of compounds comprising the group 
as component, and correspondingly to the use of compound I Bxe2x80x2 having an S-configuration for the preparation of compounds comprising the group 
as component.
Compounds (IAxe2x80x2), in particular, are valuable intermediates for the preparation of ACE-inhibitors or of their further precursors. In a number of effective ACE-inhibitors, see e.g. EP-A-50 850 and EP-A-72 352, the pharmacophoric group is defined as partial structure 
xe2x80x83having an S-configuration. This group is present in known ACE-inhibitors called benazepril, enalapril, cilazapril, spirapril, quinapril, ramipril and lisinopril. The pharmacological effect of ACE-inhibitors is described in numerous textbooks of pharmacology and pharmaceutical chemistry, inter alia in Helwig/Otto, Arzneimittel, Vol. I, 30-127-131, Wissenschaftliche Verlagsgesellschaft mbH Stuttgart 1995.
The introduction of the amino group, see the above partial structure, is carried out by known processes, for example by esterification of the HPB ester at the hydroxyl group via a sulfonyl group, for example trifluoromethanesulfonyl, so that this hydroxyl group is substituted by a leaving group. This derivative is then reacted, with inversion, with an amino compound which comprises the group xe2x80x94NHxe2x80x94R (R=additional substituents characterising the ACE-inhibitor), so that an S-configurated compound is obtained. Further processing to pharmacologically active ACE-inhibitors is described in EP-A-206 993. In detail, these further processing steps have the advantage that the compounds (I) can be reacted in the respective desired configuration (IA) or (IB) with amino compounds without any disadvantageous racemisation or without the occurrence of elimination products.
The further processing of the HPB esters to insecticides of suitable structure can be carried out by the method described in the British patent specification No. 1 014 243.
Compounds (I A) and (IB), wherein R1 is hydroxy, etherified hydroxy or C1-C4alkyl, R2 is hydrogen or C1-C4alkyl and n is 0 or 1, are valuable starting materials for the synthesis, e.g. D-maleic acid having an xe2x80x9cunnaturalxe2x80x9d configuration, or intermediates for the synthesis of correspondingly configurated amino acids, or can be used as ligand formers in complex chemistry, in particular the diketones of the compounds (I A) or (I B), wherein n is 1.
The preparation of xcex1,xcex3-diketo esters or of their tautomeric xcex1-unsaturated xcex1-hydroxy-xcex3-keto esters (II) can be carried out in analogy to the classic method described by C. Beyer and L. Claisen in Berichte, Vol. XX (1887) 2178-2188.